Hall-effect sensors are commonly used to monitor the radial position of the spindle motor of a hard disk drive. The radial position information is provided to a controller to operate a motor commutator at the correct time, to thereby maintain rotation of the motor and disks thereon at a predetermined rotational velocity. These Hall-effect sensors added additional cost and circuit board space to the hard drive system. Consequently a simpler and less expensive technique was developed using the back electromotive force (BEMF) generated by the motor with a six-state commutation. The six-state commutator operates a three-phase spindle motor to drive one winding high, one winding low, and one winding is left undriven. The BEMF voltage of the undriven winding provides radial motor position and speed information to the controller.
There are some drawbacks with the six-state commutation. Torque ripple leads to audible resonance and reduced efficiency, causing the motor speed to vary, which can also create a force on the disk drive assembly. If there is a mechanical resonance of the disk drive assembly that is close in frequency to the harmonics of this force, an audible noise results which can affect the acoustic performance of the drive. Driving the three windings of the motor with appropriate waveforms can minimize torque ripple. If the torque profile for each stator motor coil is sinusoidal, each motor terminal should be driven with a sinusoidal signal to create a flat torque waveform. A sinusoidal driving current can be accomplished by pulse width modulating the DC source voltage during each of the three phases. Sinusoidal pulse width modulation requires all three windings be driven simultaneously, with one winding being driven high and the other two windings being modulated by driving them high or low or not at all to shape the driving signal. See, for example, T. Kenjo, Electric Motors and Their Controls, Oxford Press, (1991), pp 143-136. However, with all three windings being driven simultaneously, it is not possible to detect the rotational position of the motor. Consequently, motor position information must be obtained from some other source.
One technique for detecting motor position with pulse width modulated driven motors is to sample the maximum motor current when only one of the windings is driven high and using that sample to detect the phase of the motor current in relation to the driving signal. To assure sampling is of the maximum motor current, motor current sampling is performed only when two windings are driven low. However, the sample signal carries a DC component which adversely affects operation of the apparatus. Consequently, there is a need for a phase detector that eliminates the DC component of the sample signal.